Low-pressure steam turbine and method for operating thereof

ABSTRACT

The disclosure relates to a multi-stage low-pressure steam turbine and a method for operating thereof. The steam turbine includes a last stage in which the leading edge of each vane of the last stage is skewed so as to form a W shaped K-distribution across the span of the vanes. This shape allows for efficient last stage operation at low last stage exit velocities. The disclosure includes a method for operating such a steam turbine at a last stage exit velocity between 125 m/s and 150 m/s.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to Italian PatentApplication No. MI2010A001447 filed in Italy on Jul. 30, 2010, theentire content of which is hereby incorporated by reference in itsentirety.

FIELD

The present disclosure relates to low-pressure steam turbines foraddressing exhaust losses through last stage design.

BACKGROUND INFORMATION

In the field of low-pressure steam turbines there is a desire to reduceexhaust losses in order increase turbine net efficiency. One way ofachieving this is to increase the efficiency of the last turbine stage.

As described in http://www.powermag.com/issues/coverstories/The-long-and-short-of-last-stage-blades 483 p3.html (14 Jul.2010) it is known that above last stage exit velocities of approximately190 m/s (600 ft/sec), exhaust losses in low-pressure steam turbine canincrease in a squared relationship with velocity. Although there mayseem to be a general benefit in reducing exit velocities, when reducedto below approximately 170 m/s (550 ft/sec), net efficiency can decreasedue, for example, to the increasing influence of reverse vortices at theblade root. This conclusion is further supported by, for example, K.Kavney et al “Steam turbine 34.5 Inch Low-Pressure Section Upgrades” GEEnergy GER-4269 (08/06), where, on page 7, it concludes that generallyexhaust losses are at their minimum when the exhaust velocity is about600 ft/s (190 m/s).

As described in EP 1260674 (A1) turbine stages can be classified asbeing either impulse or reaction stages depending on the blade rootreaction degree. In this context, blade root reaction degree can bedefined as the ratio of the heat drop (variation of enthalpy) across amoving blade to the total heat drop across a turbine stage. Impulseblade stages, for example, can have a blade root reaction degree ofbetween 0 to 10%. For reaction blade stages, the blade root reactiondegree can rise to about 50%. Stages with blade root degrees of between10% and 50% can be classified as low-reaction type stages. Methods ofachieving a particular blade root reaction degree are both varied andknown and can be achieved, for example, by modifying the blades lean orsweep.

Depending on the design of a stage, the blade root reaction degree mayinfluence overall blade efficiency. For example, U.S. ApplicationPublication No. 2004/0071544 describes an impulse type stage that hasimproved efficiency through adaptation of the reaction degree.

SUMMARY

A multi-stage low-pressure steam turbine according to the disclosurehaving a last stage, comprising a plurality of stationary vanescircumferentially distributed to form a vane row wherein each vane hasan airfoil with a span extending from a radially extending a base to atip of the airfoil; a plurality of blades circumferentially mounted anddistributed on a rotatable rotor of the steam turbine; and a K ratiodefined as a ratio of vane throat to pitch, where each of the vanesincludes a leading edge that is skewed so as to form a W shapedK-distribution across the span of the vanes.

A method is disclosed for operating a steam turbine including a stagehaving a plurality of stationary vanes circumferentially distributed toform a vane row wherein each vane has an airfoil with a span extendingfrom a radially extending a base to a tip of the airfoil; a plurality ofblades circumferentially mounted and distributed on a rotatable rotor ofthe steam turbine; and a K ratio defined as the ratio of vane throat topitch, where each of the vanes includes a leading edge that is skewed soas to form a W shaped K-distribution across the span of the vanes; themethod comprising configuring the area of the exit region of the laststage for operation at an exit velocity of between 125 m/s and 150 m/s;and adjusting the feed flow rate through the steam turbine such that theexit velocity is between 125 m/s and 150 m/s.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, exemplary embodiments of the disclosure are describedmore fully hereinafter with reference to the accompanying drawings, inwhich:

FIG. 1 is a sectional view of part of an exemplary embodiment oflow-pressure steam turbine;

FIGS. 2 a and 2 b show perspective views of an exemplary embodiment oflast stage vane of the steam turbine of FIG. 1;

FIG. 3 shows a top view of two last stage vanes of an exemplaryembodiment of vane row of the steam turbine of FIG. 2; and

FIG. 4 is a graph showing the K-distribution across the span of anexemplary embodiment of vane.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure are now described withreference to the drawings, wherein like reference numerals are used torefer to like elements throughout. In the following description, forpurposes of explanation, numerous specific details are set forth toprovide a thorough understanding of the disclosure. However, the presentdisclosure may be practiced without these specific details, and is notlimited to the exemplary embodiments disclosed herein.

A low-pressure steam turbine last stage is disclosed that can overcomelosses due to reverse vortices resulting from low last stage exitvelocity.

The disclosure is based on providing a low-pressure steam turbine withlast stage vanes that have leading edges skewed so as to form a W shapedK-distribution across the span of each of the vanes. This configurationprovides a means to adapt the last stage for efficient operation at lowexit velocities.

In exemplary embodiments, the last stage can be configured as a lowreaction stage through'a combination of vane 14 and blade 12 parametersincluding the tangential lean angle 30, the tilt angle of the trailingedge in the meridional plane 32 and the vane pitch 26. A reaction degreeconfiguration between impulse type blading and high root reactione.g. >50% provides further means of delaying efficiency collapse whileoperating at low exit velocities.

An exemplary method is provided for operating at low exit velocitiesbetween 125 m/s and 150 m/s. This can be achieved by adjusting the steamrate through a steam turbine with a configured exit annulus area thathas skewed vane leading edges that form a W shaped K-distribution acrossthe span of each vane and can have a last stage root reaction degree ofbetween 15%-50%.

Other exemplary embodiments further include vanes with straight trailingedges.

FIG. 1 shows an exemplary low-pressure multiple stage turbine steamturbine 10. Each stage 16 of the low-pressure steam turbine 10 includesa plurality of stationary vanes 14 that are circumferentiallydistributed on the inner casing 15 to form a vane row, and a pluralityof blades 12 that are circumferentially mounted and distributed on arotating rotor 17. The pressure of the steam exiting the last stage 18of the low-pressure steam turbine 10 defines the steam turbine as alow-pressure steam turbine 10. Typically, at steady state conditions,the last stage 18 exit pressure of a low-pressure steam turbine 10ranges from atmospheric pressure to a low vacuum.

Immediately downstream of the last stage 18, the inner casing 15 formsan annulus (not shown). The area of the annulus, together with thevolumetric flow rate of steam exiting the last stage 18 defines the exitvelocity of the steam turbine 10.

As shown in FIGS. 2 a and 2 b, each vane 14 has an airfoil 31 with aleading edge 20 and a span 36 defined as a radial extension between abase 38 and a tip 34 of the airfoil 31. As shown in FIG. 3, the distancebetween adjacent vanes 14 taken from the trailing edge 22 of one of thevanes 14 to the face of an adjacent vane 14 defines a throat 24. Thedistance between leading edges 20 of two adjacent vanes 14 defines thepitch 26. The ratio of the throat to pitch further defines a K-value.

In an exemplary embodiment, the leading edge 20 of each vane 14 isskewed so as to form, as shown in FIG. 2 b and FIG. 4, a W shapedK-distribution across the span 36 of the vanes 14.

In an exemplary embodiment, the skewed vanes 14 can be configured tohave a root reaction degree, in operation, between 15% and 50%, forexample, between 35% and 45%. As shown in FIG. 2 a, in a vane 14 of anexemplary embodiment, the trailing edge 22 has a title angle in themeridional plane 32 and, as shown in FIG. 2 b, a tangential lean angle30. The root reaction degree of exemplary embodiments can be defined bya combination of vane 14 and blade 12 parameters including thetangential lean angle 30, the tilt angle of the trailing edge in themeridional plane 32 and the vane pitch 26.

In an exemplary embodiment, the tangential lean angle 30 can be between16 degrees and 25 degrees, for example, about 19 degrees.

In an exemplary embodiment, the tilt angle 32 can be between 3 degreesand 13 degrees, for example, about 8 degrees.

In an exemplary embodiment, as shown in FIG. 2 b, each vane 14 furtherhas a tapering axial width across the span 36.

In an exemplary embodiment, each vane 14 further has a straight trailingedge 22.

Another exemplary embodiment relates to a method for operating a steamturbine 10 of any of the previously described steam turbines 10. Themethod involves configuring the area of the exit region of the laststage 18 for operation at an exit velocity of between 125 m/s and 150m/s and adjusting the feed flow rate through the steam turbine 10 suchthat the exit velocity is between 125 m/s and 150 m/s.

Thus, it will be appreciated by those skilled in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restricted. The scope of the invention isindicated by the appended claims rather than the foregoing descriptionand all changes that come within the meaning and range and equivalencethereof are intended to be embraced therein.

REFERENCE SIGNS

-   10 Low-pressure steam turbine-   12 Blade-   14 Vane-   15 Inner Casing-   16 Stage-   17 Rotor-   18 Last stage-   Leading edge-   22 Trailing edge-   24 Throat-   26 Pitch-   30 Tangential lean angle-   31 Airfoil-   32 Tilt angle in the meridional plane-   Tip-   36 Span-   38 Base

1. A multi-stage low-pressure steam turbine having a last stage,comprising: a plurality of stationary vanes circumferentiallydistributed to form a vane row wherein each vane has an airfoil with aspan extending radially from a base to a tip of the airfoil; a pluralityof blades circumferentially mounted and distributed on a rotatable rotorof the steam turbine; and a K ratio defined as a ratio of vane throat topitch, where each of the vanes include a leading edge that is skewed soas to form a W shaped K-distribution across the span of the vanes. 2.The steam turbine of claim 1, wherein the last stage is configured tohave a root reaction degree, in operation, of between 15% and 50%. 3.The steam turbine of claim 2, wherein the root reaction degree isbetween 35% and 45%.
 4. The steam turbine of claim 2, wherein theairfoil of each vane comprises: a tangential lean angle; and a trailingedge tilt angle in a meridional plane, wherein the root reaction degreeis defined by the lean angle, the tilt angle and a vane pitch.
 5. Thesteam turbine of claim 4, wherein the tangential lean angle is between16 degrees and 25 degrees.
 6. The steam turbine of claim 5, wherein thetangential lean angle is 19 degrees.
 7. The steam turbine of claim 2,wherein the tilt angle is between 3 degrees and 13 degrees.
 8. The steamturbine of claim 7, wherein the tilt angle is 8 degrees.
 9. The steamturbine of claim 1, wherein each vane has a tapering axial width acrossthe span.
 10. The steam turbine of claim 1, wherein each vane has astraight trailing edge.
 11. The steam turbine of claim 3, wherein theairfoil of each vane comprises: a tangential lean angle; and a trailingedge tilt angle in a meridional plane, wherein the root reaction degreeis defined by the lean angle, the tilt angle and a vane pitch.
 12. Thesteam turbine of claim 3, wherein the tilt angle is between 3 degreesand 13 degrees.
 13. The steam turbine of claim 4, wherein the tilt angleis between 3 degrees and 13 degrees.
 14. The steam turbine of claim 8,wherein each vane further has a tapering axial width across the span.15. The steam turbine of claim 2, wherein each vane further has astraight trailing edge.
 16. The steam turbine of claim 3, wherein eachvane further has a straight trailing edge.
 17. The steam turbine ofclaim 4, wherein each vane further has a straight trailing edge.
 18. Thesteam turbine of claim 5, wherein each vane further has a straighttrailing edge.
 19. The steam turbine of claim 6, wherein each vanefurther has a straight trailing edge.
 20. A method for operating a steamturbine which includes a last stage having a plurality of stationaryvanes circumferentially distributed to form a vane row wherein each vanehas an airfoil with a span extending radially from a base to a tip ofthe airfoil, a plurality of blades circumferentially mounted anddistributed on a rotatable rotor of the steam turbine, and a K ratiodefined as the ratio of vane throat to pitch, where each of the vanesincludes a leading edge that is skewed so as to form a W shapedK-distribution across the span of the vanes, the method comprising:configuring an area of an exit region of the last stage for operation atan exit velocity of between 125 m/s and 150 m/s; and adjusting a feedflow rate through the steam turbine such that the exit velocity isbetween 125 m/s and 150 m/s.